Centrifuge systems are routinely used to separate a biological or chemical sample into components of the sample. The sample is supported within a centrifuge rotor that is driven at a velocity to cause the components to separate according to molecular weight.
While some heating of a centrifuge rotor occurs as a result of thermal conduction from a drive motor to the motor via a drive shaft, in most applications rotor heating occurs primarily by thermal conduction from the air or other gas within the chamber, with the gas being heated by the work done on the gas by the rotor. This work takes the form of accelerating the gas and inducing a pumping action that then leads to rapid recirculation of the gas and a buildup of heat. "Windage" is primarily the power consumed in pumping the gaseous atmosphere surrounding the rotor. At high speeds, viscous frictional drag plays the role of providing mechanical coupling of the rotor to the mass of gas, resulting in the gas pumping. Thus, windage is distinguishable from inertial drag, which is dependent upon the mass and radial distribution of the rotor.
In addition to causing rotor heating, windage imposes a limitation on the maximum speed that can be achieved by a given drive motor. One approach to reducing windage is to enclose the rotor in a chamber and at least partially evacuate the chamber. Windage power varies with the mass of the gas within the chamber, the dimensions of the rotor, and the rotational speed of the rotor. The evacuation of gas within the chamber reduces the adverse effect of windage on accelerating the rotor, but has mixed results on allowing temperature control. While a reduction in generation of thermal energy follows from the evacuation of gas, a partially evacuated environment causes the thermal energy that is generated to remain localized about the rotor. It is known to use refrigeration coils around the exterior of the housing that forms the chamber, but the heat must be transferred from the rotor to the housing in order for the refrigeration system to provide the desired cooling. The evacuation renders convection heat transfer from the rotor to the housing less effective, so that a greater reliance is placed on heat transfer by irradiation.
Operating a vacuum system at a level in which the chamber is fully evacuated is often viewed as being undesirable. The required vacuum system is expensive. Moreover, fully evacuating the chamber greatly increases the evaporation rate of sample solution, thereby adversely affecting the analytical procedure, particularly for applications in which a rotor and specimen tubes remain open to the evacuated chamber.
U.S. Pat. No. 4,857,811 to Barrett et al. describes a centrifuge in which identification of a rotor within a centrifuge is used to control vacuum. Upon identification of the rotor, information regarding the identified rotor is extracted from a look-up table of data related to various rotors that may be operated with the centrifuge. A vacuum system is controlled in accordance with the information. Optionally, the rotational speed of the rotor can be monitored and the vacuum level can be changed with changes in speed.
One difficulty with an approach such as the one taken by Barrett et al. is that the vacuum control is memory intensive. Data must be stored for each possible rotor. Another difficulty is that the memory must be upgraded with each addition of a rotor compatible with the centrifuge system.
Optimum operation of the vacuum system and/or the refrigeration system of a centrifuge will vary based upon the rotor in use. This dependence of optional settings upon the rotor in use also applies to other process parameters. For example, optimal drive proportional and integral gain values for a centrifuge drive system will vary according to the inertia of the rotor. A feedback loop control for a drive system should utilize values that preferably are adjusted when a rotor is substituted with a rotor having a different inertia.
It is an object of the present invention to provide a centrifuge system and method which is adaptive to substitutions of rotors without requiring the adaptive circuitry to maintain data regarding each possible rotor available for use with the centrifuge system.